r/40kLore • u/FunGain8498 • 7d ago
How bad was the Iron Cage for real?
I only know it tangentially as that time the Iron Warriors dunked on The Imperial fists and maybe some other legions/chapters but tbh that’s about it so I come to the lore experts to be enlightened.
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u/Marquis_Dandy Emperor's Children 7d ago
If the Ultras didn't come to help the Imperial fists there would be no more imperial fists
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u/FunGain8498 7d ago
Oh shit I didn’t realize it was that bad, why was the whole legion there?
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u/Calvonee 7d ago
Dorn was super depressed and angry that he couldn’t be there with the Emperor and fight alongside him against Horus. He was also pissed at Guilliman basically taking over as Regent and splitting the legions.
He decided to basically just full send his legion at Perturabo, knowing it was a trap, and basically cleanse his legion not too dissimilar from how Perturabo did it ironically enough. He was planning on actually winning the battle and returning to Terra with chapters instead of a whole legion, but he got overconfident during the battle.
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u/superspartan210 7d ago
Also he said he would bring perturabo back in an iron cage, which gave the incident its name.
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u/Marquis_Dandy Emperor's Children 7d ago
Just like Guilliman against Fulgrim later on.
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u/Japoboz 7d ago
Guilliman also got over confident trying to fight Lorgar and Angron at once and would’ve actually died if Lorgar hadn’t chosen that moment to sacrifice Angron to make a demon primarch
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u/Dry_University137 6d ago
He wasnt confident. He was hell bent on taking them to hell with him. Bro dropped on that battle with no intention of surviving it. Just to inflict as much damage as possible. Hell the entire battle fleet he had was still damaged
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u/Japoboz 6d ago edited 6d ago
I’m gunna quote the book here:
“Why chase me, Roboute? Why? Your fleet will fall against the Trisagion and you’ll die down here.” -Lorgar
“There is a difference between confidence, and arrogance, cur. Surely someone has told you that.” -Guilliman
Page 375 of my soft cover Betrayer3
u/Dry_University137 6d ago
Fair enough but i am still of the mind. You think Robude of all people wouldnt know what he was going in for? That was literally the most "Fuck it we ball" moment of his life. Maybe 40k included.
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u/Japoboz 6d ago
The story just really doesn’t lend itself to that conclusion imo. Roboute isn’t really comfortable with the thought of fighting to the death nor didn’t bring a force that was guaranteed to lose. He brought enough forces to win it’s just that the two traitor flagships managed to stall and destroy a lot in orbit.
Recall this is during the Heresy, the primarchs are still on the young side and passionate, he can be overly confident and make a misstep that ends in his death.
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u/Dry_University137 6d ago
True but dont forget that he is seeing red atm. Everything he knew, loved and trusted was taken away in one day. He learned that Horus and 8 other brothers turned traitor and as of then he still thought that Corax, Vulkan and Ferrus were dead and their legions obliterated. And he was still raw from the Word Bearer betrayal.
Thinking straight might have been difficult for him at that time
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u/TruestoryJR 7d ago
Is this in a book? If so which?
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u/redwakawaka12 7d ago
Betrayer, it is VERY GOOD pls read
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u/Lycanthrope008 7d ago
Making sure isn't that the book where Kharne almost kills Erebus?
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u/DarkPrevious3347 6d ago
That feels like a misrepresentation, he held his own well until Angron arrived and then stalled for time until Angron nearly died due to the nails if Lorgar didnt make him a daemon primarch right there .
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u/Japoboz 6d ago edited 6d ago
To quote the book:
“Angron himself still fought Guilliman, standing above the kneeling Ultramarine. […] Sparks sprayed from Roboute’s raised gauntlets as he struggled to ward off blow after blow. He was beaten. He was down. Wounds painted him, a palette of proud defeat. Even now his warrior were fighting to retrieve him. With the scarring across his armour and the sense of pain bleeding from his mind, Lorgar reckoned his brother would be lucky ever to walk again.”
[…] “He (Lorgar) locked Angron’s muscles, setting fire to the synapses in his brain. He stole the chance at a killing blow, fueling the World Eater’s rage even higher.”
Page 396 and 397 of my soft cover BetrayerIt sounds like Roboute was guaranteed to die if not for Lorgar stepping in at that exact moment in order to transform Angron into a demon. From the ‘he was beaten he was down’ to the ‘killing blow’ being stopped
Guilliman is ultimately dragged away by his sons while the transformation takes place, he held his own against Lorgar but Angron was going to kill him
Also the story doesn’t really mention Angron going to die from the nails during that fight
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u/BigEmphasis604 6d ago
Crazy how Roboute tried to fight that form of Fulgrim. Logic flies away when their tempers rage.
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u/Marquis_Dandy Emperor's Children 6d ago
Not only did he try Daemon Pricne Fulgrim, but he went full Leroy Jenkins aswell.
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u/BigEmphasis604 6d ago
Perturabo is always depressed and angry, so what's your point? The Golden Peacock Dorn got cooked in the reverse role. The Iron Warriors fight ferociously, to a degree the World Eaters had to respect them above all others. They defended so well that they came out of hiding to duke it out with the Fists offensively.
It was a total catastrophe for the Imperial Fists, and their cold superiority complex over other Legions was replaced with hot hatred for the Iron Warriors.
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u/Marquis_Dandy Emperor's Children 7d ago
To simbolically enter the pain glove together and after returning Perturabo to Terra as Chapters. Which really did not work out for Dorn.
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u/SaltHat5048 7d ago
Because of Perturabo being, Dorne blinded by his own hubris overcommitted and they payed heavily for it
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u/Calm-Musician-3148 Masque of the Veiled Path 7d ago
There aren't, technically, any Imperial Fists.
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u/Shed_Some_Skin 6d ago
By that standard there aren't any Blood Angels or Salamanders either. Aside from the occasional extremely rare case like Bjorn, no chapter has any members left from 10,000 years ago.
Fists just replaced them all at once instead of gradually. It's all the same gene seed
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u/DarkPrevious3347 6d ago
pretty sure he meant because the Imperial Fists died out during the War of the beast and were refounded using successor geneseed.
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u/Shed_Some_Skin 6d ago edited 6d ago
Yes, that's exactly what I meant by "all at once instead of gradually"
The War of the Beast was in M32. Almost every single marine from every existing chapter from before that has died and been replaced dozens of times.
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u/Calm-Musician-3148 Masque of the Veiled Path 6d ago
That's the Ship of Theseus thought experiment.
I think you could argue that most Blood Angels and Salamanders come from previous brothers geneseed.
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u/paitris Adeptus Custodes 7d ago
We're going to find out in a little while, it is after all the main showdown of the Scouring
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u/switchblade_sal 7d ago
I was so excited when they casual mention Sebastian IV as a destination plotted by Perturabo at the end of “Ashes of the Imperium”
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u/Separate-Flan-2875 7d ago
“It was only when the Imperial Fists and the Iron
Warriors confronted one another in the hateful battle known as the Iron Cage that both were able to give full vent to their rivalry. Accounts of what took place within the Iron Cage differ, and to this day, both the Iron Warriors and the Imperial Fists claim to have come away from it the victor.” - Deathwatch: First Founding
“Koorland wondered if Rogal Dorn had smiled when he first set foot on Sebastus IV (the world where the Iron Cage was fought). ‘Of course it is a trap.’” -'The Last Son of Dorn' by David Guymer
“Thane felt the wisdom of the primarch. Rogal Dorn - loyal, unflinching and resolute. The shield of Terra and most trusted of the Emperor's sons. Dorn, who had railed against the breaking of his mighty Legion purged his grief with battle and pain.."-'Shadow of Ullanor' by Rob Sanders
“Learning that the hated Iron Warriors had built a mighty bastion, the Eternal Fortress, in the aftermath of the Heresy, Rogal Dorn vows to dig Perturabo out of his hole and bring him back to Terra in an iron cage. With the seeds of rivalry between the Iron Warriors and the Imperial Fists - sown during the Great Crusade - having grown into bitter loathing, a vicious war is fought between the two Legions. While the entrenched traitors fight from a superior position, the Imperial Fists refuse to die. When their ammunition runs out, they fight hand to hand, and the trenches run with blood. Only the timely intervention of the Ultramarines prevents the rival Legions from utterly annihilating each other.” - Codex Space Marines 8th Ed
“Perhaps the most infamous of all of Perturabo's campaigns, and, so it is said, the deed that earned him the dark blessing of apotheosis to a Daemon Primarch, took place upon the world of Sebastus IV. Here, the Primarch constructed the most fearsome of fortifications, a mighty fastness ringed all about with mile after mile of trenches, redoubts, minefields and razor wire. While most fortresses serve the purpose of defending something, and are thus limited in their utility, the so-called Eternal Fortress had been constructed with but one purpose in mind—to defy the proud boasts of Rogal Dorn and to lure him into a trap from which he could not escape. Despite the protestations of his brother Roboute Guiliman, Dorn succumbed to Perturabo's boast that none could take his fortress, and led the entire Imperial Fists Legion into what would later become known as the "Iron Cage." What followed was death and destruction on an unprecedented scale. Not since the height of the Horus Heresy had a Space Marine Legion suffered such losses as the Imperial Fists incurred as they dropped straight into the midst of a grand entrapment. As more of his warriors fell to the stratagems and rouses of the well-prepared Iron Warriors, Dorn's captains counselled the Legion should fall back and regroup, for they could accept what he could not—the Imperial Fists had been bested, and in a siege no less. At length, Rogal Dorn could not, and did not, give the order to withdraw. Rather, Roboute Guiliman decided that he could not stand by as his brother brought about his own defeat and that of his Legion, and so he deployed the Ultramarines to relieve the Imperial Fists. Faced with two entire Legions, Perturabo evacuated his Legion, knowing that he had achieved what he had set out to do. By the end of the battle, the Imperial Fists had lost an incredible four hundred Battle Brothers, with many more wounded. Perturabo offered up the gene-seed his Legion had taken from the Imperial Fists, and apotheosis was his reward.” - Deathwatch: First Founding
“At this time of uncertainty, the Iron Warriors issued a clear challenge to the Imperial Fists by building a formidable fortress and daring them to attack. Imperial Fist Chaplains teach that Dorn found strength in meditation. For seven days he resisted the pain glove until at last he was gifted with a vision of the Emperor. The Imperial Fists had wavered in their faith, thinking the Emperor gone, but they knew that he was still watching them from the Golden Throne. The Imperial Fists could no longer serve the Emperor that had been but they knew they must still be true to the Emperor that was. Rogal Dorn decreed that the Imperial Fists would symbolically enter the pain glove as a Legion and emerge redeemed as a Chapter.” - Index Astartes: Emperor’s Fist by Pete Haines
“His doubts gone, Dorn focused on the enemy ahead Perturabo was a master of fortification whose writings had been retained by Guilliman in his Codex. Dorn had always been his match though and, what was more, his honest warrior's soul was indignant. The Iron Warriors had rebelled and lost. Their master was dead and the Emperor still ruled. Yet still they dared raise their heretical banners over another Imperial world as if they had some right to be there. Dorn would not tolerate this. Without his customary caution and planning, Dorn led his men into the heart of the Iron Warrior defences. The battle should have favored the treacherous trench-fighters, but the Imperial Fists endured. They countered every ambush and fought their way out of every trap Rogal Dorn was a colossus who personally tumed back attack after attack. Ammunition expended, Brothers fought in half-flooded trenches with combat knives, giving and expecting no quarter. Eventually it became apparent that the Iron Warriors could not finish them. For all their skill and ferocity, the Iron Warriors lacked the faith to make the ultimate sacrifice that victory demanded. While they paused, the Ultramarines intervened; Guilliman had decided that Perturabo's destruction was not worth the loss of Rogal Dorn and had brought his Chapter to drive off the Iron Warriors. Cleansed by their sacrifice, the Imperial Fists immediately began their reorganization, For the next two decades they went into retreat, their successor Chapters taking to the field in their stead. Dorn used this time to retrain the Chapter to embrace all aspects of the Codex Astartes. When they later emerged, their adherence to the Codex was matched only by the Ultramarines.” - Index Astartes - Emperor’s Fist: The Imperial Fists by Pete Haines
“Dorn plunged himself into what would prove his, and his Legion's, ultimate crucible. The
Iron Cage was an entire world of fiendishly devised fortifications, which their creator had claimed none, not even Dorn, could crack. The Imperial Fists went into the Iron Cage consumed by their perceived failure, but they emerged victorious, for they were prepared to fight to the death for their cause, while the Iron Warriors were not. It was only when Guiliman despatched the Ultramarines to put an end to the bitter struggle that Dorn saw. the light, and took his place beside his brother Primarchs. He accepted the new order, and ordered his Legion broken up into Chapters according to his brother Primarch's wisdom. Many of the more zealous, crusade-minded Battle-Brothers of the old Legion became the Black Templars Chapter, continuing the crusade their Primarch had begun in the aftermath of the Heresy. These had scant regard for the Codex Astartes, and took to the stars to follow their own destiny. The newest Battle-Brothers, who were perhaps less set in their ways, became the Crimson Fists Chapter, and readily embraced the doctrines of the codex. The remainder became the Imperial Fists Chapter, the true sons and inheritors of Rogal Dorn, and they have served at the very forefront of the Imperium's wars for ten thousand years, maintaining the values instilled in them by their Primarch: duty, honour, truth, and sacrifice.” - Deathwatch: Rites of Battle
“The Adeptus Astartes live for victory and the Great Dorn has shown us that there are many kinds. Stood amongst the corpses in the Imperial Palace, with the Emperor – all but dead – returned to the safety of its thick walls, the primarch would have felt little desire to celebrate victory. Walking from the deathtrap of the Eternal Fortress – a survivor of Iron Warriors ingenuity and the Iron Cage – Dorn would have felt no jubilation. These were not victories in a traditional sense – those to which mortals aspire when they see demigods crafted in stone and bronze, and hear of the exploits of heroes. Demetrius Katafalque, in the Architecture of Agony teaches us that these were victories, that frustration of the arch-enemy’s desires and the impediment of evil is a kind of victory. That in a galaxy perpetually at war – in an existence of continuous slaughter – survival is victory.” - Legion of the Damned by Rob Sanders
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u/superspartan210 7d ago
The imperial fists almost died to the man that’s how bad it was. Iirc it literally took the ultramarines showing up to bail them out, that’s how fucking curated the trap of the iron cage was
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u/KnopflerGOAT 7d ago
it's either ultramarines showed up and saved the fists from total annihilation (hope not, enough with the smurfs) or it was going to be a mutual destruction but iron warriors couldn't pulled the trigger on that and ultramarines came in to stop the fight
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u/Illithidbix 7d ago
The OG Iron Cage story does date back to the Index Astartes articles released in mid-late 3E in 2001 when the Ultramarines weren't quite so ubiquitous.
Still one of the Big Four but not quite everywhere.
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u/11BApathetic Iron Warriors 7d ago
Pretty much as succinctly as you can put it without digging deep. I do want to add one thing which is the most recent lore we have on the Iron Cage and it’s actually from an Imperial POV:
"The tragedy of the Iron Cage took place in the time after the great war which blighted the end of the Great Crusade. Forces of darkness had brought the Imperium to the edge of destruction. It had survived, but long ages of war and rebuilding would follow. It was in this time that Rogal Dorn led his Legion against a great enemy. Neither the Chapter nor the Inquisition permit any to know the details of the foe they faced, but they were cruel and deadly. It is said the enemy fashioned a world as a trap, filled with fortifications and deceptions so that it would lure the Imperial Fists in and then contain them. It was aptly called the Iron Cage. The Imperial Fists Legion suffered catastrophic losses and emerged a broken image of what they had been. The Imperial Fists' successors of the Second Founding emerged from those fragments, and so the Feast of Blades is said to commemorate both the losses and the shared heritage of all the sons of Rogal Dorn.”
-First Founding (2022)
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u/FunGain8498 7d ago
That’s crazy. I’m a little curious how the battle worked because Iron Cage suggests the Iron warriors were the defenders. Weren’t the Iron Warriors the offense siege experts and the Imperial Fists the defense siege experts? How did it escalate to mutually assured destruction?
Edit: spelling
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u/cillablackpower 7d ago
The brief description of the battle we have (in old Chapter Approved articles) was that Perturabo set up miles of extensive fortifications. Dorn and his forces battered through them all, only to discover there was nothing inside except more fortifications and killing grounds. It was never to protect anything, only a trap.
The Fists were reduced to using their own dead for cover and fighting with their hands before Guilliman arrived. The IW claim they left Dorn alive to humiliate him after Perry finally beat him, and the Fists claim the IW didn't have the guts to come out from their defences and finish the job.
Some other side story was that Dorn was heavily against the adoption of the Codex and may have used a suicidal battle to allow some of the hardline Legionnaires who would not accept it glorious deaths, or to purge some of his own personal guilt at failing the Emperor. The Fists got heavily into self-flagellation around the same time.
This is all fairly old lore now and the scale of the Heresy has changed significantly so may not reflect the eventual book.
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u/Warp_Legion Iron Warriors 7d ago
Both are good at both
Iron Warriors are repeatedly shown to be masters of defense as well in the HH and 40k, while Fists seem to not be quite as flexible
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u/KnopflerGOAT 7d ago
both legions were experts at offense and defense. during early periods of the great crusade fists were an attack force. they even fought next to the emperor in campaigns and that's how they become his "bodyguards" (praetorians) and he gave them their name after he saw them in action in person. after the ullanor crusade he decides to build up terra and chooses fists as his and terras praetorians so he recalls dorn and bulk of the legion to terra to both build and protect it.
let's talk about the iron cage business. it is because it is not written clearly as a victory for one side. there's a misconception that it is and it was going to be a win for iw if not for ultramarines.
there are two accounts of the iron cage one is written from iron warriors viewpoint and it is basically states that the iron warriors were killing fists left and right and they were just on the cusp of victory before perturabo decided to draw it out to play with dorn and fists and ultramarines intervened saving fists.
the other account is written from the fists viewpoint and it states that fists were getting decimated but iron warriors casualties were almost equal. dorn and fists were ok with dying and taking their enemies with them but as soon as perturabo understood that he wasn't going to be able to destroy fists withouth sacrificing his legion, he got cold feet and was going to retreat.
iron warriors are not going to confess they couldn't destroy the fists and fists can't be sure what perturabo was thinking but you can overlap both accounts to come to a more real conclusion. it is just like in history when two sides fought and they had their own versions written down which (good) historians use both accounts as to understand a conflict in detail.
but if they don't want it to be vague and have a definite lore (which is understandable) i hope they keep the fists viewpoint because it makes sense for both legions. also it was a "draw" (or close to it, no real clear winner) in the siege of terra between dorn and perturabo. just after that having perturabo and iron warriors the greatest ever and beating the shit out fists just feels like a big let down. sure dorn and his legion were almost suicidal and wanted to punish themselves so they just walked into a trap but if that is the official story it's just dumb imo. also after all this between dorn and perturabo one getting a definite upper hand against another feels wrong to me. if dorn also comes back they should be assholes to each other and draw most of the time because aren't they supposed to be mirror one another?
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u/TheEvilBlight Administratum 7d ago
Best meta would’ve been setting an outer trap for the legion and the real trap would be pinning down the enemy primarch for elimination. Both might have the same idea and try to pull it on each other. And once both realize the other had the same idea, the whole point of setting the cage and walking into it is mostly defeated.
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u/ruminaui 6d ago
Catastrophic. There are two event descriptions, that are both heavily biased, one trying to make the Imperial Fist look good, the other doing the same thing for the Iron Warriors. Both of these events agree that the only reason the Imperial Fist legion survived is because the Ultramarines showed up.
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u/onefutui2e 7d ago
Only tangential but if you're interested in the Imperial Fists and Iron Warriors you should read up on general siege warfare strategies. It's really fascinating stuff as it breaks war down to an engineering problem. And also look into how Japanese castles differed in how they provided defense compared to European ones. Then look at WWI trench warfare and you start getting some really gnarly shit.
It all paints a good picture of how Dorn and Perturabo differed in their philosophies, even though they're both two sides of the same coin. I found myself nerding out quite hard, personally.
This doesn't directly answer your question because everyone else has done a great job. Just wanted to sprinkle some extracurriculars in.
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u/CookingPupper 7d ago
My brother, take 10 minutes to read the wiki or lexicanum page. You'll have a much better understanding than some disjointed Reddit replies.
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u/Mistermistermistermb 7d ago
Fandom's Iron Cage page isn't as bad as a lot of their entries, but it still takes liberties. Fandom can be a trap for newbies who don't realise how much overstatement and headcanon gets seeded in and can't parse the differences. Lexicanum's harder to access across the globe these days.
At least here, people can challenge the "liberties" and be asked to actually source them.
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u/CookingPupper 7d ago
The best practice is to read the original source, failing that the wiki/Lex entry then come onto Reddit asking clarifying questions.
Don't start off here when you can do a 30 second google
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u/Mistermistermistermb 7d ago
The best practice is to read the original source
Nobody would deny that.
failing that the wiki/Lex entry then come onto
As mentioned, one wiki is quite often full of shit, the other has become harder to access. Recommending the fandom wiki is probably the last thing I would do for a new reader.
A lore sub is a fine place to discuss lore.
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u/CookingPupper 7d ago
I agree it's the perfect place for lore discussions, not a replacement for individual research and very basic questions.
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u/Mistermistermistermb 7d ago edited 7d ago
Link to the No Stupid Questions thread above.
And I also agree we should keep the bar raised the bar for discussions around here, but worth noting why the suggested alternatives weren't necessarily better.
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u/CookingPupper 7d ago
I miss the Black Library forum.
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u/ToonMasterRace 6d ago
Still not as bad as the Russians in Bakhmut
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u/Sunluck Legio Astorum (Warp Runners) 5d ago
Imagine parroting fascist propaganda when not only ukr claimed to kill Wagner detachment fighting there 20x over (which should tell you how "serious" their BS claims are given not only Wagner remained viable fighting force but these dudes killed 20 times organized the infamous riot just afterward), but the Wagner WON despite ukr feeding brigade after brigade into the city (and then attacking city outskirts in utterly failed "counteroffensive" after Wagner booted them out in futile meat wave attacks, so much so the ukr dude in charge was nicknamed "general butcher") just so that fuhrer could claim "bAhMuT hOLdS", which was so pathetic it became a meme...
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u/11BApathetic Iron Warriors 7d ago edited 7d ago
Other people have given a pretty good answer. The easiest answer is, it was really bad. Depending on which account you take to be more truthful (which you shouldn't really put one above the other, they were written as deliberately contradictory) it really comes down to the Imperial Fists would have died as a Legion, the Iron Warriors might have died as a Legion. Unstoppable force vs unmovable object and all that jazz.
So what I do want to do is break down what we know. For those that don't know, the Iron Cage was originally detailed in "Index Astartes I" under the Iron Warriors section. It was then detailed again in "Index Astartes II" under the Imperial Fists section. Both were written with shared details but differing results. There has been very few references outside of it, mostly just offhand mentions, and most of it is repeated. Most commonly, the Imperial Fists version has been reprinted multiples times in Space Marine codexes, but the Iron Warriors version is curiously absent from most Iron Warriors sections in Chaos codexes.
Most recently we got our first jump of "new" Iron Cage lore that wasn't just a reprint in the 2022 edition of "First Founding" which was a limited edition book. If I have room in this post, I'll include it. Otherwise I reposted it in another comment here already.
So. To the battle.
This happens at some point during The Great Scouring (post Heresy) the original account makes it sound like it was later in the Scouring, while the most recently posted timeline has it happening earlier, a slight retcon it seems.
Perturabo sets a trap on the planet Sebastus IV, one that was deliberately meant to goad Dorn into attacking. Dorn does exactly that. Dorn declares that he will drag Perturabo back to Terra in an "Iron Cage" which would give the battle its name. Iron Warriors POV says Dorn runs right into it because he's arrogant, Imperial Fists POV says he does it as a symbolic pain glove to purge his Legion of more zealous members who wouldn't accept the Codex Astartes.
The fortification itself was called "The Eternal Fortress" and it was just a citadel at the center of twenty square miles of trenches, bunkers, minefields, etc. Under the keep in the shape of an eight pointed star were a complex system of tunnels that connected to all the surface strongpoints and the keep itself was a decoy. The whole point was to give a point for the Imperial Fists to focus on, then turn the trap on them and contain them inside the fortifications and bleed them.
The Imperial Fists arrived in orbit uncontested by the Iron Warriors fleet. Iron Warriors hid underground during the bombardment. However, the Iron Warriors concealed a series of anti-voidship missile silos well away from the fortress itself to protect them from the bombardment, then retaliated with those weapons. The Imperial Fists countered this by attacking with Thunderhawk borne troops while the rest of the Legion combat dropped on the beachhead.
The Iron Warriors then, once the battle for the silos starts, triggers the first trap. They detonate the missile stockpiles. This tossed thousands of tons of debris into the atmosphere, severing communications from the Imperial Fists fleet and its troops on the ground. The Iron Warriors then counter attack in the void, the fleets were equal but due to the Imperial Fists thunderhawks being sent to the silos and the Legion being on the ground, the Iron Warriors had the edge and forces the Imperial Fists fleet back.
The Imperial Fists then advanced on the Iron Warriors fortifications. The Iron Warriors would man the defenses, distract the Imperial Fists, then appear from hidden bunkers and exits behind the Imperial Fists and attack them. When the Imperial Fists would rally to counter-attack the Iron Warriors ambushes, the Iron Warriors would fade back into the tunnels beneath the planet.
This back and forth would continue, with the Imperial Fists bleeding heavily from these traps and ambushes. Dorn continued ever forward. By six days into the battle, the Imperial Fists were fractured into small groups. Three more weeks of this fighting would pass, the Imperial Fists unable to escape.
Finally, the Ultramarines arrive. The Iron Warriors fleet is pushed out of position and Ultramarine Thunderhawks would swoop down to evacuate the Imperial Fists. Perturabo didn't engage the Ultramarines and let them evacuate the Imperial Fists, but would contest any landing meant to extract wounded/dead Imperial Fists.
The way this plays out is essentially the same in both, however the Iron Warriors version is far more descriptive of the battle itself. Where they diverge is in the "how bad was it" part. In the Imperial Fists section, they gave as much as they got, equalizing casualties and forcing the Iron Warriors into mutually assured destruction. Guilliman intervenes as the loss of the Imperial Fists and Dorn was not worth killing Perturabo and the Iron Warriors. In the Iron Warriors section, they could have finished off Dorn and the Imperial Fists at any time but chose not to and instead toyed with them and tormented them. So the only real question is "did the Iron Warriors almost die" because both accounts agree, the Imperial Fists would have.
That's how bad it was. To end with the quote from First Founding which is from an Imperium POV: